Imagine a box where there is a live cat, a Geiger counter measuring the decay of a atom in which after one hour the atom has 50% chance of decaying. If it happens, the Geiger counter will detect it and set off a mechanism which will break a vial of poison gas which will kill the cat. Or it might not and the cat lives. Before opening the box, is the cat dead, alive or both dead AND alive?
This was the apparent paradox of quantum physics and in particular, quantum entanglement. Posed as a paradox to the Copenhagen interpretation, it slowly turned to being an example of Quantum Superposition. Before going into any details, I shall briefly explain what this is and what it meant to do.
Copenhagen interpretation is the interpretation of how quantum mechanics affected everyday life. One particular part in which this thought experiment aimed to contradict was Quantum Entanglement. For example, say two subatomic particles that interacted. Now if you move them a million miles apart, Quantum Entanglement says that they are not in a definite state until one of them is measured, in which they will have a definite state. One example of this is the spin of the particle. Before measuring either one, they can be both 'up' and 'down' spin, which in reality there can only be one way they spin. According to Quantum Entanglement, once you measured the spin of one to be up, then the other particle will immediately 'make up its mind' and have the opposite spin.
Now some people didn't like the idea of things being connected and all and one of them was Erwin Schrödinger. Thus he formulated this thought experiment to challenge Copenhagen interpretation. First, we look at what Copenhagen interpretation has to say about this.
According to Copenhagen interpretation, the cat is in a state of superposition until observed. This means that the cat is in all its possible states, here being dead and alive, until someone opens the box. In Quantum Mechanics, this is what happens to most particles. Imagine shining a torch at two slits, with each slit being very small. Now light is made of individual packets of energy called photons. What happens is the photons, before us observing it, will take every possible path through the two slits. It could go through one hole, fly around the universe before going through the second hole for all we know. If we were to isolate one photon and look at that one photon, it could be said that the photon was in every possible state. But when we observe that one photon, all its states collapse into one and we would measure the photon to only be in one location and only taking one path.
Another interpretation was the Many Worlds Interpretation. In this interpretation, instead of the cat being either dead or alive when we observe it, it actually exists in both states, just that they are not able to interact with each other. In other words, when the box is opened, the observer with the dead/alive cat is split into the observer with a live cat in the box and a observer with a dead cat in the box. Both scenarios happen, just in different branches of the universe. Think about this as a parallel universe.
This was the apparent paradox of quantum physics and in particular, quantum entanglement. Posed as a paradox to the Copenhagen interpretation, it slowly turned to being an example of Quantum Superposition. Before going into any details, I shall briefly explain what this is and what it meant to do.
Copenhagen interpretation is the interpretation of how quantum mechanics affected everyday life. One particular part in which this thought experiment aimed to contradict was Quantum Entanglement. For example, say two subatomic particles that interacted. Now if you move them a million miles apart, Quantum Entanglement says that they are not in a definite state until one of them is measured, in which they will have a definite state. One example of this is the spin of the particle. Before measuring either one, they can be both 'up' and 'down' spin, which in reality there can only be one way they spin. According to Quantum Entanglement, once you measured the spin of one to be up, then the other particle will immediately 'make up its mind' and have the opposite spin.
Now some people didn't like the idea of things being connected and all and one of them was Erwin Schrödinger. Thus he formulated this thought experiment to challenge Copenhagen interpretation. First, we look at what Copenhagen interpretation has to say about this.
According to Copenhagen interpretation, the cat is in a state of superposition until observed. This means that the cat is in all its possible states, here being dead and alive, until someone opens the box. In Quantum Mechanics, this is what happens to most particles. Imagine shining a torch at two slits, with each slit being very small. Now light is made of individual packets of energy called photons. What happens is the photons, before us observing it, will take every possible path through the two slits. It could go through one hole, fly around the universe before going through the second hole for all we know. If we were to isolate one photon and look at that one photon, it could be said that the photon was in every possible state. But when we observe that one photon, all its states collapse into one and we would measure the photon to only be in one location and only taking one path.
Another interpretation was the Many Worlds Interpretation. In this interpretation, instead of the cat being either dead or alive when we observe it, it actually exists in both states, just that they are not able to interact with each other. In other words, when the box is opened, the observer with the dead/alive cat is split into the observer with a live cat in the box and a observer with a dead cat in the box. Both scenarios happen, just in different branches of the universe. Think about this as a parallel universe.
There are many more ways to 'solve' this paradox and they are listed on http://en.wikipedia.org/wiki/Schr%C3%B6dinger's_cat .